This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
In a field of antenna technology a low frequency differential antenna has been difficult to realize in practice. For low frequency single ended (unbalanced) antennas, the ground plane is the main radiator and currents are induced in it by the antenna wire which acts as an excitation element, so below 1 GHz the size of the ground plane has a large effect on performance of an unbalanced antenna. But in an idealized low frequency differential (balanced) antenna the current flows only in the wire/resonating element and not in the ground plane so the size of the radiating element becomes more important to the antenna performance. For a truly balanced 900 MHz antenna the radiating element would be quite large. Because modern consumer electronics such as mobile terminals are very constrained in size, there is a desire to find an alternative arrangement for a balanced antenna that has efficient performance in both the low and high frequency bands and which is small enough to integrate effectively with other close-packed electronics within a handheld device.
As radio spectrum becomes more scarce and the ability to pack multiple radios into a single handset continues to improve, the need for low frequency communications using a mobile user device is growing. Low frequencies in this context refers to frequencies below about 1000 MHz. While different regions and countries define their radio spectrum differently, in some areas the 900 MHz band is in the Industrial/Scientific/Medical ISM band or television whitespace TV WS bands (and thus is license-exempt spectrum), in others it was once used for the Global System for Mobile Communications GSM radio access technology, and it is also the band used by the ZigBee radio access technology. Generally low frequency refers to the lower half of the UHF band, roughly about 200 MHz to about 1000 MHz.